For purposes of geologic exploration, and especially in connection with exploring the earth for underground petrochemical fossil deposits, it is known to measure certain electrical properties existing along an earth-formation wall. Typically a bore hole is formed, as by a conventional rig, and with the bore hole filled with a drilling mud, an electrode structure on a pad is moved along the length of the bore hole while a voltage is applied between electrodes and pad, and the resultant currents and/or voltages present at the wall are monitored and recorded. Recording may be on magnetic tape, on a ink recorder, on a computer disk or in a computer memory, as examples. A simultaneous record of the position of the electrode structure along the wall is made, so that a plot of distance along the bore hole against a selected electrical property may be formed from the stored data.
The variations in the electrode currents and/or voltages thus monitored and displayed have been found to correlate with certain properties of the adjacent earth formation. One way in which this correlation can be determined is to form a hole in the earth by coring--that is, a narrow cylindrical cut is made into the earth and the core inside the cut removed intact, to leave the bore hole. Logging of the exposed bore hole wall is then performed, and the resultant measured electrical values at each depth compared with the structure and properties of the core at corresponding locations. Experience obtained with such coring correlations enables one to attribute specific physical, chemical and geologic properties to the values and shapes of logging waveforms which are obtained later in other bore holes, with a high degree of certainty.
More particularly, a popular technique for obtaining the desired resistivity measurements is to use an array of electrodes or "buttons" mounted on, but insulated from, an electrically conductive pad, which is urged against the bore hole wall as it is dragged upwardly, while a voltage or current level is applied between each button and a remote electrode in the mud or at the surface. The currents thereby produced flow at least in part from the electrodes through the bore hole wall, in a magnitude dependent upon the properties of the earth formation immediately adjacent the electrodes. Such procedures have been termed microresistivity measurements, since they measure the electrical resistivity of very small vertical segments of the wall structure.
While a separate logging waveform may be obtained for each electrode by such resistivity measurement, it has been found highly advantageous to convert the information producing such resistivity waveforms into a bi-dimensional visual image of the resistivity of the track along-which the array is pulled. This image typically exhibits brightness variations corresponding to variations in resistivity of the strip or segment of wall which is traversed by the electrode array. One form of such system is the Formation Micro-Scanner (FMS.TM.) of Schlumberger Well Services, and has proved to be commercially successful for bore hole logging in the course of petrochemical exploration. Information relating to such systems is contained, for example, in U.S. Pat. Nos. 4,468,623 of Gianzero et al, issued Aug. 28, 1984, and 4,567,759 of Ekstrom et al, issued Feb. 4, 1986. One drawback of such systems is that they tend not to work well with oil-based muds, which have high resistivities. Since oil-based muds are highly desirable in some cases for reasons not related to the logging function, this limitation constitutes a substantial drawback in resistivity logging.
Furthermore, in some cases the resistivities of strata of quite different materials or physical configuration may not differ substantially from each other; for example, strata having very different sand and shale content may exhibit about the same resistivity, making their differences undetectable by micro-resistivity logging.
Another known procedure for obtaining indications of certain gross characteristics of the earth formation is to log the spontaneous potential (SP) of the exposed surface of a bore hole in an earth formation by pulling an electrode upwardly in the bore hole and using the measured electrical potential at the electrode as an indication of SP. The measured SP may then be displayed as a plot of SP versus distance along the bore hole. The SP electrode in such cases is typically mounted near the center of the bore hole, and while it produces some useful indications of gross variations in the nature and structure of the adjacent earth, the definition of which this arrangement is capable is typically quite poor, e.g. of the order of a foot. This has been adequate for the traditional purposes to which SP measurement has been directed, i.e. to distinguish thick shale formations from other formations, such as sand strata, and to determine formation water conductivity. However, in the evaluation of thinly laminated reservoirs, it is desirable to have resolutions of the order of 0.1 inch.
It is also know to conduct the logging of SP along a bore-hole wall by using an electrode recessed slightly below a surface of an electrically-insulating pad, and to pull the pad upwardly while the pad surface is urged against the bore-hole wall. While the resultant logging waveform was of some utility, it did not find wide general use because it did not produce sufficiently reliable, accurate, clear representations of SP in the adjacent earth formation.
The SP signal has two components: the electrochemical potential and the electrokinetic potential. The electrochemical potential arises from the diffusion of ions between the formation water and the drilling fluid under an ionic concentration difference. The electrokinetic potential arises from the movement of fluid from the borehole to the formation under a pressure difference. The electrochemical potential has been traditionally used to determine the formation water conductivity (related to formation water ionic concentration), an important parameter in formation evaluation. It can potentially be used to determine cation exchange capacity in shaly sand, again an important parameter in formation evaluation of shaly sand reservoirs. The electrokinetic potential may be used to monitor the movement of fluid through low permeability zones.
The strength of the source SP signal can be controlled only by changing the pressure or the ionic concentration of the borehole fluid. Such changes are impractical under most drilling conditions. In comparison, the voltages and currents injected during the resistivity type of measurements previously described can easily be adjusted by the operator to nearly any convenient level to produce an optimum level of measured signal. The prior ineffectiveness of conventional SP logging procedures for monitoring fine structure is, at least in part, due to the fact that strength of the SP cannot be easily controlled and the measurement is therefore especially vulnerable to noise.
Specifically, there are four important kinds of measurement noises and distortions which have affected SP measurements:
(1) changes in the potential of the reference electrode; PA0 (2) polarization of the electrodes; PA0 (3) bimetal currents; and PA0 (4) the ohmic potential drop from the flow of the SP current in the borehole.
These four different kinds of noise may be explained as follows:
(1) The reference electrode is typically placed in the mud pit, or on the surface. The potential of this reference electrode can change during logging due to surface phenomena (weather, for example).
(2) All the measurement electrodes are made of metals which are subject to polarization effects. The measured potential between such electrodes therefore depends on surface states of electrodes which can change during logging.
(3) All tools have exposed metal parts. Any two different metals exposed to the drilling fluid form a battery, and the "bi-metal current" flowing between these two metals distorts the SP measurement.
(4) The borehole fluid is typically substantially conductive. SP currents flow in a closed loop; part of the loop is in the formation and part of the loop is in the borehole fluid. Conventionally, SP is measured with an electrode located in the center of the borehole which measures the ohmic drop in potential from the current flowing in the borehole. The measured SP is therefore only a part of the total potential (SSP, or Static SP). In the presence of many thinly layered beds, the ohmic drop in potential in the center of the borehole therefore depends in a complex way on the SSP along a substantial length of the formation, resulting in a signal of low definition. It is not possible to estimate the high definition SSP from the measured low definition ohmic drop in the center of the well.
It has been proposed to mount an SP-sensing electrode so that it is urged outwardly to bear directly against a surface of a bore hole as it is pulled upwardly, in an effort to improve the SP measurements. See for example U.S. Pat. No. 3,914,686 of R.H. Brooks, issued Oct. 21, 1975. However, to the best of applicant's knowledge, the particular SP electrode arrangement of the cited Brooks patent has never been used commercially, and it is now clear that it could not work properly as described in that patent, since the SP electrode will change drastically and unpredictably as it is pulled along the bore wall, in direct contact with it.
It is also known to use a pair of vertically-aligned electrodes positioned near the center of the borehole to produce absolute and relative SP values, and to combine the low-frequency components of the absolute SP with the high-frequency components of the relative SP's to obtain more accurate SP measurement. While helpful in this regard, SP has still remained a parameter of little commercial significance or value, especially with respect to obtaining indications of thin layers of differing SP's measured along the length of the bore hole.
Accordingly, an object of this invention is to provide a new and improved system and method for determining the geologic fine structure of earth formations by measuring spontaneous potential (SP).
Another object is to provide such an SP system and method which provide reliable indications of the fine structure of an exposed surface of an earth formation.
A further object is to provide such SP system and method in which SP fine structure can be discerned readily and accurately, even in the presence of substantial interfering, spurious signals or conditions, especially to distinguish between sand and shale strata or different degrees of shaliness.
It is a further object to provide such a method and apparatus which operates satisfactorily even with bore-hole muds of relatively high resistivities, such as oil-based muds.